Chemical mechanical polishing (CMP) is widely known in the semiconductor fabrication industry. CMP pads are used to planarize wafers after some other wafer fabrication process has been performed. Some CMP pads are non-porous, such as the solid and grooved model OXP 3000 manufactured by Rodel. Other CMP pads have continuous porosity throughout the entire pad, such as Cabot Microelectronics' Epic model, which is formed of polyurethane, or Rodel's Suba IV model, which is formed of interlocking felt fiber. Continuous porosity means that there are pores throughout the pad, and the pores are interconnected. Still other CMP pads have isolated porosity, such as Rodel's IC1000 and Rhodes' ESM-U. Isolated porosity means that while pores may be located throughout the pad, the pores are not interconnected.
A problem encountered with continuously porous CMP pads is that a higher level of wafer defects is experienced when compared with non-porous pads. As an example of this, a shallow trench isolation (STI) polish and a polish on borophosphosilicate glass (BPSG) layer polish were performed with the continuously porous Cabot Epic pad. While several important polishing characteristics were found to be good, the proportion and severity of scratches on the wafers was unacceptably high. For the BPSG layer polish, the defect levels were on an order of magnitude difference compared to expected defect levels.
In general, however, continuously porous pads are more desirable than non-porous pads. Porous pads have a rough surface texture which is beneficial to polishing, since it promotes slurry transport and provides localized slurry contact. As porous pads wear, the homogeneous porosity allows a similar texture with polish and conditioning to be maintained, since a new, porous, rough surface is constantly being regenerated.
It is believed that the higher level of defects from conventional continuously porous CMP pads may be due to a lack of sufficient hydrodynamic lift during the polishing process. With reference to FIGS. 1-3, a wafer 10 is illustrated juxtaposed with a continuously porous CMP pad 14. A slurry 12 is transported in a direction A relative to the wafer 10 and the pad 14. Some of the slurry 12 infiltrates pores 16 of the pad 14. As a force is directed against the wafer 10 in a direction B, the slurry 12 tends to further migrate in a direction C into the pores 16 of the pad 14. This prevents the building up of a sufficient hydrodynamic lift in the slurry 12, causing large slurry particles 18 to contact the wafer with increased force (FIG. 3).
FIG. 4 illustrates a non-porous CMP pad 30 with grooves 32. During polishing, pressure builds up in the slurry 12, creating a hydrodynamic lift in a direction D. FIG. 5 shows a CMP pad 40 with isolated pores 42. As polishing commences, a hydrodynamic lift is created in a direction E in the slurry 12. Both hydrodynamic lifts D and E illustrated in respectively FIGS. 4 and 5 assist in suppressing the force with which slurry particles, including the large slurry particles 18, strike the wafer 10.
There is therefore a need for a CMP pad which has the advantages of a continuously porous pad without its attendant disadvantages.